A typical power cable generally comprises one or more conductors in a cable core that is surrounded by several layers of polymeric material including a first semiconducting shield layer, an insulating layer, a second semiconducting shield layer, a metallic tape or wire shield, and a jacket. Thermosetting power cable compositions are currently produced via one of two types of off-line processes in which polymerization, mixing, and vulcanization are carried out independently. A thermoplastic polymer is supplied to the mixer in the form of pellets (granules), fluff, or powder. The purpose of the mixer is to incorporate functional and stabilizing additives into the polymer. Then, the polymer/additive mixture is subjected to vulcanization where crosslinking agents are added. A variant of the off-line production process is practiced when the thermoplastic polymer/additive mixture is mixed with a crosslinking agent in a mixer/extruder during fabrication of the power cable. This process is referred to as the direct injection process and combines the vulcanization step with the fabrication step. In both of these processes, filtration is effected, somewhat inefficiently because of low throughput rates. Due to decomposition and the reaction of the crosslinking agent in the fabricating extruder, periodic shutdowns of the extruder must be made to remove crosslinked product (scorch) from the wire. Thus, the aforementioned processes suffer from inefficiency and a lack of cleanliness.
A thermosetting polymer is a polymer capable of being changed into a substantially infusible or insoluble product by the addition of heat or radiation wherein the primary change to the polymer is chemical. The net effect of the chemical change is to link the polymer chains thereby increasing the polymer's weight average molecular weight. A thermoplastic polymer, on the other hand, is one which is capable of being softened by heating and hardened by cooling through a characteristic temperature range. For polyolefins, that temperature range is typically about 80 to about 170 degrees C. The change to the polymer on heating is substantially physical.
All power cable products including semiconductive power shields and dielectric insulation materials are valued for their purity or freedom from unintended additives, which are considered to be contaminants. Since the power cable will be exposed to high voltages, minute extraneous particles such as metal, inorganic oxides, salts, and polar organic substances, when permitted to be present in the power cable tend to cause deterioration, disrupt the insulation, and shorten the service life of the power cable. Contaminants to power cable compounds include any organic or inorganic substance not specifically detailed in the product formulation. This includes very highly crosslinked polymer and thermally or oxidatively degraded polymer particles in the final product. Power cable insulation is partially specified and sold by identifying the number and size of contaminants in a given size range contained in a given mass of product. The size ranges specified for these contaminants vary by product and geographic market, typically beginning at 0.004 inches or 100 microns. There is an industrial need for thermosetting power cable compositions, which have a low level of contaminants, both in number and size.